3.5 stars due to the non-intuitiveness of the endings
Erika ending - most important things are to be as nice to her as possible, then sympathize with sitting alone, then the second time you study with her at home "ask her what she means". Make sure she doesn't go to the hanami. It's okay if she goes to the hike though.
Hana ending - be nice to her, but don't actually do anything with her. Follow the Erika path. Then, make sure that Erika goes to the hanami, and talk to her there. Switch back to the Hana path at the hanami and hike. The answers to her two questions are Love and Time Travel. So it seems like one of the keys to Hana is to play Erika against her until the hike.
R ending - key thing is to follow the Erika path, then ask her to sit down for a drink, then make fun of her.
Haven't found any of the other endings yet.